1. Field of the Invention
The present invention relates to an austenitic heat-resistant cast steel, in particular, to an austenitic heat-resistant cast steel excellent in the thermal fatigue characteristics.
2. Description of Related Art
An austenitic heat-resistant cast steel has been used for exhaust system parts and so on for a vehicle such as an exhaust manifold, a turbine housing and the like. Such components are exposed to a high temperature and severe use environment. In order for the components to have excellent thermal fatigue characteristics, it is considered necessary to be excellent in the high-temperature strength characteristics and toughness from room temperature to a high temperature.
From such a viewpoint, for example, Japanese Patent Application Publication No. 07-228950 (JP 07-228950 A) proposes an austenitic heat-resistant cast steel that includes 0.2 to 0.6% by mass of C, 2% by mass or less of Si, 2% by mass or less of Mn, 8 to 20% by mass of Ni, 15 to 30% by mass of Cr, 0.2 to 1% by mass of Nb, 1 to 6% by mass of W, 0.01 to 0.3% by mass of N, and the balance of Fe and inevitable impurities. Such a heat-resistant cast steel is obtained in such a manner that a molten metal obtained by melting a material containing the components described above as a starting material is heat-treated under heating condition of 1000° C. and 2 hours to remove residual stress after casting.
Further, Japanese Patent Application Publication No. 06-256908 (JP 06-256908 A) proposes a heat-resistant cast steel that has a composition consisting of 0.20 to 0.60% by mass of C, 2.0% by mass or less of Si, 1.0% by mass or less of Mn, 4.0 to 6.0% by mass of Ni, 20.0 to 30.0% by mass of Cr, 1.0 to 5.0% by mass of W, 0.2 to 1.0% by mass of Nb, 0.05 to 0.2% by mass of N, and the balance of Fe and inevitable impurities. The heat-resistant cast steel has a two-phase structure of 20 to 95% of an austenite phase and the remainder of a ferrite phase.
However, since the austenitic heat-resistant cast steel described in JP 07-228950 A contains austenite crystal grains in a large part of the structure, while tensile strength at high temperatures is high, since austenite crystal grains are excessively contained, the thermal expansion coefficient is large and the thermal fatigue characteristics are insufficient.
On the other hand, since the heat-resistant cast steel described in JP 06-256908 A is a two-phase heat-resistant cast steel of an austenite phase and a ferrite phase, the thermal expansion due to austenite crystal grains such as described above can be reduced. However, the ferrite phase itself is present in the structure as crystal grains. Therefore, due to ferrite crystal grains softer than the austenite crystal grains, the tensile strength at high temperatures is not high. Thus, while the heat-resistant cast steel described in. JP 06-256908 A suppresses the thermal expansion, the tensile strength at high temperatures is smaller than that of a conventional austenitic heat-resistant cast steel and, as a result, the thermal fatigue characteristics were insufficient.